1. Field of the Invention
The present invention relates to a cassette. More particularly, the present invention relates to a cassette for holding substrates inside a load-lock.
2. Description of the Related Art
In general, in a fabrication of a semiconductor product or an electro-optical device, a number of processing steps must be performed with some of the processing steps being performed in a vacuum state. To prevent a reaction chamber from getting exposed to air, a wafer or substrate undergoing a reaction in a vacuum state is often placed inside a load-lock first. The load-lock is a special area where wafers or substrates are free to move in and out of the reaction chamber without exposing the reaction chamber to the atmosphere. A trap door is normally installed somewhere before or after the entrance to the reaction chamber so that the reaction chamber is always maintained in a vacuum state through the trap door and a vacuum exhaust system.
Inside the load-lock, the substrates or wafers that require to be processed with a vacuum reaction are placed inside a substrate cassette or a wafer cassette. In order to carry out the reaction, a robotic arm retrieves a substrate or a wafer from the substrate cassette or the wafer cassette and then transports it to the reaction chamber one at a time.
FIG. 1 is a schematic diagram showing the design of a conventional cassette inside a load-lock. As shown in FIG. 1, the outer portion of the cassette is labeled 10 and the inner portion of the cassette is labeled 20. The cassette comprises an outer casing 102, a plurality of braces 106 and a plurality of side plates 108.
A front surface 140 of the outer casing 102 of the cassette has a plurality of slots 100 for inserting substrates into and for retrieving substrates out of the cassette. In most opto-electronic manufacturing facilities, each cassette has 12 slots 100 for holding 12 pieces of substrate.
In the interior 20 of the cassette, there are four braces 106 and a plurality of side plates 108. The braces 106 are positioned at the four edge corners of the outer casing 102 and the side plates 108 are fastened to the braces 106. For each slot 100, all four braces 106 have a side plate 108. In other words, each slot 100 has four corresponding side plates 108 for supporting a substrate 200 (as shown in FIG. 2).
Furthermore, as shown in FIG. 3, there are four rectangular holes 104 at the bottom section of two sidewalls 130a, 130b of the outer casing 102. These rectangular holes 104 are designed to facilitate a tool to enter into the interior of the cassette and fasten the horizontal screws.
FIG. 4 is a sketch showing the side plate latching onto the bottom section of the outer casing inside the rectangular hole. After a substrate is processed and sent back to the cassette inside the load-lock, the robotic arm will move into the cassette to retrieve another substrate for processing. However, due to thermal suction and vacuum electrostatic effect, the side plate 108 is often sucked up by the substrate 200. The sucked-up side plate 108 may easily lodge inside the rectangular hole 104 at the bottom section of the sidewalls 130a, 130b. When the robotic arm tries to retrieve a substrate 200 from the cassette, the substrate 200 may collide with the stuck side plate 108 inside the rectangular hole 104 leading to some damages. Such occurrences will obstruct production resulting in a drop in productivity. Since the rectangular holes 104 are formed on the bottom section of the sidewalls 130a, 130b of the outer casing 102, the lodging of the side plate 108 within the rectangular hole 104 mostly occurs when a substrate is retrieved from the slot 100 at the very bottom of the cassette.